Sintering method



United States Patent Qfifice 3,199,735 ?atented Nov. 5, 1963 3,199,735 SENTERING METHUD .iohn M. Googin, Oak Ridge, Tenn, assigner to the United States of America as represented by the United States Atomic Energy Commission No Drawing. Filed Oct. 30, 1961, Ser. No. 148,763 4 Claims. (Cl. 75224) This invention relates to an improved method of sintering powder metallurgy compacts.

A typical powder metallurgy process comprises: preparation of metal powders; compacting the powders into a desired shape at either room or elevated temperatures; and sintering the compacted article at elevated temperatures, usually in the environs of a protective or reducing atmosphere, to promote bonding of the individual particles. The resultant product may be further treated to improve its properties, or worked into a more useful configuration.

The sintering step of the powder metallurgy process is extremely important because it is in this step that the article acquires its strength, density, and coherence. Although the ultimate properties of an article produced by powder metallurgy techniques are strongly dependent on such factors as powder size and shape, the nature of the materials used, and the temperature and time of treatment, the purity of the powders is perhaps the most important requisite of fabricating high quality articles. One deleterious impurity in powders is oxygen, which may be present in either the oxide form or as adsorbed oxygen. In either form, oxygen hinders proper bonding of particles causing an inferior product of low density to result from the sintering operation.

In the prior art, the problems caused by impurities have, to a large part, been avoided by careful preparation of powders and either utilization of these powders shortly after preparation or storage of the powders under a protective atmosphere until they are needed. Impurities which remain, notably oxygen, have been removed by conducting the sintering operation under a reducing atmosphere such as hydrogen or carbon monoxide. However, for reasons which will be given below, this technique has not been completely satisfactory, particularly in the fabrication of relatively thick articles. Apparently the impurities are not completely reduced or the reaction products such as CO and H are not removed from the interior of the sintered article.

The deficiencies of present sintering methods have been more noticeable in recent years due to the application of powder metallurgy techniques to the fabrication of components for rockets and nuclear reactors. Temperatures in these devices are extremely high necessitating the use of refractory metals such as tungsten which can be prepared efiiciently by powder metallurgy methods. Temperatures attained in gases passing through rocket nozzles exceed 5000 F., and virtually unlimited temperatures may be obtained in nuclear reactors. It is highly desirable for articles used in these devices to have the highest densities and strengths obtainable. Thus, any deficiency in powder metallurgy techniques constitutes a limitation on the performance of these devices.

As a general rule, the density and strength of an article produced by powder metallurgy techniques increase with a decrease in powder size, all other factors being equal. Unfortunately, however, the oxygen content of powders increases as the powder size decreases. For this reason, the use of ultrafine powders to obtain the high densities required in the rocket and nuclear fields has been hampered.

The oxygen content of tungsten powders which have particles in the 2 to 5 micron range is usually of the order of 0.05 percent or less. The ultrafine powders used in some of these investigations have particle diameters as low as 0.05 micron and, depending on the degree of care used in the handling operations after reduction, may have several tenths of one percent oxygen. While the oxygen contamination is not necessarily present, as a practical matter there is always some and it is this which the present procedure is designed to compensate for.

Moreover, as another general rule, other factors being equal, the greater the thickness of an article to be sintered, the more difficult it becomes to remove impurities during the sintering operation. Rocket nozzles are quite thick, and this factor, in combination with the small particle size required for high density, has made prior sintering methods unsatisfactory; either the density achieved by prior sintering methods has been unacceptably low, or the sintering time required to achieve acceptable densities has been unreasonably long.

It is, therefore, a general 'cbjectof the present invention to provide an improved method for sintering powder compacts.

Another object is to provide a sintering method for powder compacts fabricated from ultrafine powders.

A further object of the invention is to provide a sintering method which provides greater densities than prior sintering' techniques for equal sintering times.

A still further object of the invention is to provide an improved method for sintering ultrafine tungsten powders.

Other objects of the invention will become apparent from an examination of the following description of the invention and the claims appended thereto.

In accordance with the principles of the present invention, the above objects are attained by heating a powder compact to sinter it in the presence of a gas, the pressure of which is alternately increased and decreased.

The improved sintering process is applicable to all powder compacts, including ceramic compacts, and will be advantageous in any sintering operation wherein the sintering atmosphere has a beneficial effect on properties of the final product. It is especially applicable to the sintering of oxygen-contaminated, powder-metallurgy compacts under a reducing atmosphere, particularly the sintering of tungsten under hydrogen.

In carrying out my invention the pressure of the gas in contact with the article being sintered is alternately increased and decreased. The amount of the change in pressure is not critical, and even small pressure changes will increase the purification rate to some degree although the purification resulting from pressure changes involving the removal of less than about ten percent of the gas per cycle is insignificant. Relatively large pressure changes are desirable and it is preferred that the ratio of the highest pressure to the lowest pressure in a cycle be at least four or, to state it in other words, that at least seventy-five percent of the gas present at the high pressure he removed with the decrease in pressure.

The absolute value of the gas pressure is not critical and, although high pressures provide more gas for reaction, pressures normally used in sintering are suitable in my sintering method.

Generally, any rate of pressure variation will afford some improvement over static sintering. However, very high rates of change, e.g., changes greater than one per minute, are expensive to accomplish and inefficient in gas utilization, and changes of less than about one per hour allow only a few cycles during a sintering process. The preferred rate of pressure variation is in the range of one to sixty per hour, and depends to some extent on the void fraction of the compact and the amount of purifica- 3 tion desired, high void fractions allowing lower rates, and high purification requiring higher rates of cycling.

Changes in pressure, carried out during any portion of the process, will increase the purification rate. I have found, however, that unexpectedly superior results are achieved if the pressure is varied during the time the temperature of the powder compact is being raised, and in the preferred form of carrying out my invention the pressure is varied during the heating step as 'Well as during the period the powder compact is maintained at an elevated temperature.

Since the invention allows the effective use of smaller sized powders than does static sintering, and smaller sized powders are sinterable at lower temperatures, use of the invention allows sintering at temperatures lower than were used in prior static processes. For instance, whereas temperatures in prior static sintering of tungsten were in excess of 2500 C., the present invention allows sintering to proceed at 2000 C. or less. This reduction in temperature constitutes a marked economic advantage over static sintering.

As illustrative of the advantages of the present invention over static sintering, and of one suitable method of practicing the invention, the following examples are offered. Example I illustrates static sintering, while Example II is illustrative of sintering in accordance with the present invention.

Example I A bell-shaped tungsten compact having a height of ten inches, at central hole four inches in diameter, and top and bottom outer diameters of five and ten inches, respectively, was made by filling a mold with 5 micron tungsten powder and pressing at 30,000 p.s.i. at room temperature. The compact thus formed was placed in a sintering furnace under hydrogen at atmospheric pressure and the temperature raised at the rate of 400 C./hr. to a sintering temperature of 1900 C. where it was held for four hours. After approximately three hours cooling, the sintered compact was removed from the furnace and its density determined. The density was 13.2 g./ cc.

Example II A powder compact was made in the manner described in Example I from a tungsten powder mixture containing 80 wt. percent 5 micron powder and 20 wt. percent powder which was less than one micron in diameter. The resultant compact 'was sintered at the temperature and for the time specified in Example I, but the pressure of the hydrogen atmosphere was continuously varied between 5 inches and 23 inches of mercury at a rate of 3 cycles per hour. The sintered product which resulted had a density of 18.01 g./cc.

As can be seen from a comparison of the above examples, the variable pressure sintering technique provides a product having a substantially greater density than static sintering for an equal length of time will afford. Although static sinterin-g may produce densities which are comparable to the densities afforded by the invention, the time required will be considerably longer.

It will also be noted that a considerable portion of the powders used in Example 11 were u ltra'fine powders having a diameter of less than one micron. If static sintering had been used on particles this small, lower densities would have been obtained for the same processing time.

The above examples were given for illustrative purposes only and should not be interpreted in a limiting sense. Rather, the invention should be limited only by the cliams appended hereto.

What is claimed is:

1. In a method of making an article comprising the steps of packing a powder into a desired shape, raising the temperature of the resulting powder compact to a sintering temperature in the presence of a reducing gas, and maintaining the resulting heated compact at said sintering temperature long enough to sinter said compact, the improvement comprising alternately decreasing and increasing the pressure of said gas at least during the step of raising the temperature of said powder compact.

2. The improvement of claim 1 wherein the pressure of said reducing gas is alternately decreased and increas d at a rate of one to sixty cycles per hour.

3. The improvement of claim 1 wherein the ratio of the highest pressure to the lowest pressure of the gas is at least four.

4. In a method of making a tungsten article comprising the steps of packing a tungsten powder into a desired shape, raising the temperature of the resulting powder compact to a sintering temperature in the presence of reducing gas, and maintaining the resulting heated compact in the presence of said reducing gas at said temperature long enough to sinter said compact, the improvement comprising alternately increasing and decreasing the pressure of said gas throughout the course of the steps of raising the temperature of said powder compact and maintaining the heated powder compact at a sintering temperature.

References Cited in the file of this patent UNITED STATES PATENTS 2,129,844 Kiefer Sept. 13, 1938 2,263,520 Romp Nov. 18, 1941 2,776,887 Kelly et al. Jan. 8, 1957 

4. IN A METHOD OF MAKING A TUNGSTEN ARTICLE COMPRISING THE STEPS OF PACKING A TUNGSTEN POWDER INTO A DESIRED SHAPE, RAISING THE TEMPERATURE OF THE RESULTING POWDER COMPACT TO A SINTERING TEMPERATURE IN THE PRESENCE OF REDUCING GAS, AND MAINTAINING THE RESULTING HEATED COMPACT IN THE PRESENCE OF SAID REDUCING GAS AT SAID TEMPERATURE LONG ENOUGH TO SINTER SAID COMPACT, THE IMPROVEMENT COMPRISING ALTERNATELY INCREASING AND DECREASING THE PRESSURE OF SAID GAS THROUGHOUT THE COURSE OF THE STEPS OF RAISING THE TEMPERATURE OF SAID POWDER COMPACT AND MAINTAINING THE HEATED POWDER COMPACT AT A SINTERING TEMPERATURE. 